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OHI-Northeast | OHI Science | Citation policy

Summary

For OHI Global Assessments, we calculated a climatology from 1982-2017 per cell using weekly data from CorTAD. We then calculated the number of times a given cell’s weekly SST was greater than the climatological mean for that week (an anomaly: greater than mean + one standard deviation) and summed the number of weekly anomalies in a single year. The maximum value a cell could have is 52 which would mean that cell had anomalous SST temperatures for each week of the year.

To account for annual variation, we look at Sea Surface Temperature anomalies in 5 year periods, so the maximum value possible per cell is 260 anomalous weeks. To rescale the values from 0 to 1 we set a reference point. Previously, the reference point for SST has just been the maximum difference in anomalous weeks between the most recent time period and a historical reference period (1985-1989).

This time we have decided to use a reference point that represents a regime shift. Once a given cell is anomalous for more than 50% of a five-year period, it has shifted into a new regime. All cells that have a value greater than 130 weeks (51% of a 5 year time period) are assigned a 1. The rest of the cells are scaled to this reference point by dividing by 130.


Data

Source: The Coral Reef Temperature Anomaly Database Version 6 (CoRTAD)
Downloaded: August 21, 2018
Description: Sea Surface Temperature Anomalies (Kelvin)
Native data resolution: 4km2
Time range: 1982 - 2017
Format: NetCDF


Analysis

Global data layers

We’re going to use the global data that was processed for OHI 2018. This data is held on a server at NCEAS.

Each of these rasters is the number of positive weekly anomalies each year.

Crop to the Northeast region

Using the crop function from the raster package we crop all sea surface temperature rasters to our extent and then reproject them to the US Albers projection for consistency across the assessment. We crop the global rasters first to reduce the time it takes to reproject the rasters. ocean_ne is used as a mask to remove land cells from the raster for better visuals.

Rescale

To account for annual variation, we look at Sea Surface Temperature anomalies in 5 year periods, so the maximum value possible per cell is 260 anomalous weeks. To rescale the values from 0 to 1 we need to set a reference point. Previously, the reference point for SST has just been the maximum difference in anomalous weeks between the most recent time period and a historical reference period (1985-1989).

This time we have decided to use a reference point that represents a regime shift. Once a given cell is anomalous for more than 50% of a five-year period, it has shifted into a new regime. All cells that have a value greater than 130 weeks (51% of a 5 year time period) are assigned a 1. The rest of the cells are scaled to this reference point by dividing by 130.

Results

Citations

Saha, Korak; Zhao, Xuepeng; Zhang, Huai-min; Casey, Kenneth S.; Zhang, Dexin; Zhang, Yongsheng; Baker-Yeboah, Sheekela; Relph, John M.; Krishnan, Ajay; Ryan, Thomas (2018). The Coral Reef Temperature Anomaly Database (CoRTAD) Version 6 - Global, 4 km Sea Surface Temperature and Related Thermal Stress Metrics for 1982 to 2017. NOAA National Centers for Environmental Information. Dataset. https://doi.org/10.25921/ffw7-cs39. Accessed: August 21, 2018/

Selig, E.R., K.S. Casey, and J.F. Bruno (2010), New insights into global patterns of ocean temperature anomalies: implications for coral reef health and management, Global Ecology and Biogeography, DOI: 10.1111/j.1466-8238.2009.00522.x.